Today high capacity communication via optical fiber is commonly used and optical networks using optical fibers have become widespread as they are suitable for handling the rapidly growing communication of various multimedia services or similar requiring high bandwidth. On the whole it is fair to say that there is an increased interest for transporting large volumes of information with high spectral efficiency in the optical domain.
Optical transmission systems of today are therefore using advanced modulation formats, e.g. such as Quadrature Phase Shift Keying (QPSK) and 16 Quadrature Amplitude Modulation (16-QAM) and similar. Here, the information is carried in the amplitude and phase of the optical field rather than in the optical intensity as have been done before.
Normally so-called coherent receivers must be used in order to demodulate optical signals carrying information in the amplitude and phase of the optical field. In commonly known coherent receivers the incoming optical signal is mixed with the light from a Continuous Wave (CW) Local Oscillator (LO) and the electrical beat components generated upon square law photo detection in a photo detector are used as an electrical counterpart to the optical signal. However, the phase information is lost upon square law detection and there are two different ways that are typically used in order to recover both phase and amplitude of the light.
The most straight forward way to recover both phase and amplitude of the light is referred to as homodyne detection. The homodyne detection uses two parallel coherent receivers whose LO laser have 90° relative phase shift and with the LO laser frequency set to the center of the optical spectrum that is to be demodulated. The two 90° phase shifted LO laser signals must be generated from the same laser and the 90° phase shifted signals are usually generated in an optical 90° hybrid. From the two entities produced by the parallel receivers, often called in-phase signal (I) and out of phase quadrature signal (Q) components, the full phase and amplitude information can be recovered in a Digital Signal Processor (DSP).
The other way to recover both phase and amplitude of the light is usually referred to as heterodyne detection. The heterodyne detection uses one single optical LO signal placed outside the optical spectrum to be recovered and one photo detector with square law detection. In this case, the optical spectrum is converted into an Radio Frequency (RF) signal with the optical information spectrum centered at an RF frequency equal to the frequency separation between the LO and the center of the optical information spectrum. Subsequently the electrical RF signal can be down converted in the electrical domain into I and Q signals that will be equal to the I and Q signals obtained with homodyne detection described above.
FIG. 1 shows an implementation of a typical coherent polarization diversity heterodyne optical receiver 100. Before entering the receiver 100 it is preferred that a received optical signal OA is filtered by an optical filter 110. The received optical signal OA is then decomposed into two orthogonal optical polarizations by an optical polarization rotating arrangement 112 so as to form a first branch with a horizontally polarized signal and a second branch with a vertically polarized signal. The horizontally polarized signal in the first branch is then combined with an optical oscillator signal LO (e.g. at the frequency fC) in a first combiner arrangement 114a, whereas the vertically polarized signal in the second branch is combined with the optical LO-signal in a second combiner arrangement 114b. The optical oscillator signal LO may be produced by an optical oscillator 115, e.g. a suitable laser arrangement or similar. The first combined signal in the first branch is then converted to a first electrical RF-signal RFA_horiz in a first balanced optical detector 116a, whereas the second combined signal in the second branch is converted to a second electrical RF-signal RFA_vert in a second balanced optical detector 116b. A balanced optical detector may contain two photo diodes and a differential amplifier. The RF-signal RFA_horiz and the RF-signal RFA_vert are electronically demodulated into base band signals I and Q before being digitized in an Analogue to Digital Converter (ADC) and processed by a Digital Signal Processor (DSP). The RF-signal RFA_horiz and the RF-signal RFA_vert may e.g. be demodulated by a first RF-demodulator 118a and a second RF-demodulator 118b respectively.
As schematically indicated in FIG. 1, a demodulation of the RF-signal RFA_vert to a baseband signal may e.g. be accomplished by mixing the RF-signal RFA_vert with an electrical LO-signal of frequency f1 produced by an RF oscillator 145. To this end, an in-phase component I may be obtained by using a first RF mixer 147a to mix the RF-signal RFA_vert with the electrical LO-signal in-phase. A quadrature phase component Q may be obtained by using the RF oscillator 145 and a phase shifting device 149 and a second RF mixer 147b to mix the RF-signal RFA_vert with the electrical LO-signal phase shifted by 90°. The same applies mutatis mutandis to a demodulation of the RF-signal RFA_horis to a baseband signal. This is all well known to those skilled in the art and it needs no further description.
However, since the whole optical signal is converted onto an RF frequency, the bandwidth of the photo detector and subsequent electronics of an optical heterodyne receiver must be at least twice compared to the corresponding components in an optical homodyne receiver where the optical signal is split into two base band signals. Moreover, heterodyne detection has another important restriction in that any optical input signal must only appear on one side of the optical carrier frequency fLO, i.e. the other side must not contain any optical energy, preferably not even optical noise, which e.g. may require thorough filtering. Moreover, in a real optical communication system many optical wavelength channels are transmitted simultaneously, so called Wavelength Division Multiplexed systems (WDM), i.e. independent optical channels are transmitted on different optical wavelengths in the same fiber. Usually, these wavelength channels have been separated by at least 50 GHz or more and optical filters are present at both transmitters and receivers in order to avoid optical energy from different wavelength channels to mix in a receiver. Recently, there is a great interest to decrease the frequency separation between optical channels and thus the optical filters should preferably be avoided. At the same time there is still an interest in supporting simultaneous reception of multiple optical channels in one coherent receiver, though it may be difficult to use a heterodyne receiver for the reasons now mentioned.
Thus a coherent homodyne optical receiver may be preferred for receiving an optical multichannel signal. FIG. 2 is a schematic illustration of a coherent polarization diversity optical homodyne receiver 200, which will be discussed in more detail later.
However, conventional optical homodyne receivers can not detect multiple channels unless these are recovered in a Digital Signal Processor (DSP). Here the optical spectral information is converted into the I and Q electrical signals and thus one single DSP must handle the complete receiver bandwidth. In the DSP the I and Q signals are treated as real and imaginary part of a complex number and there the complete spectrum can be obtained using negative frequencies. However, in analog electronics only real signals can be manipulated and thus the complete optical spectrum is not trivially recovered.